This invention relates to automobile and light truck tire combinations designed specifically for either the front wheel position or rear wheel position of front engine four wheeled mounted vehicles.
Automobile and light truck vehicles that have front engines and front steering suspensions have a vehicle weight distribution that is heavily loaded on the front position tires and lightly loaded on the rear position tires.
Light truck tires are routinely driven with no weight in the bed of the vehicle causing the rear tire position to typically operate at 50% of the tires rated load. When the truck is carrying weight the load can be increased up to 100% of the tires rated load on the rear tires.
Mini-vans and sport utility vehicles in addition to being weight distribution sensitive have higher centers of gravity than automobiles.
These multi purpose vehicles (MPV) put greater demands on tires due to their higher center of gravity and non-uniform weight distribution. This combination causes the vehicle to undergo greater amounts of vehicle roll putting higher loads on the outside shoulders of the front tires with resulting increases in wear rates. Goodyear developed the Wrangler GS-A and Wrangler Aquatred to meet these demands by engineering a tread design with distinct tread zones for specific performance demands. The outside shoulder was solidified to resist the tendency for fast front tire shoulder wear while providing traction through tread blocks in the other zones of the tread. This design approach provided an: improved level of treadwear and traction for MPV""s in all wheel positions.
Tires for the front wheel positions of MPV""s are subjected to special demands because of the higher center of gravity of the vehicle and the greater tendency for the vehicle to roll onto the outside shoulders of the tire. Looking at the footprint patch of the prior art tire in the front right wheel position (as depicted in FIG. 1) one notes the higher outside contact area of the shoulder 2. The tire geometry has been designed by increasing the tread mass distribution in this portion of the tread to resist the higher presure and abrasion. The remainder of the tread area is optimized for traction and hydroplaning resistance. This state of the art design approach is embodied in the Wrangler GS-A and Wrangler Aquatred tires and represents the present state of the art in tread designs for MPV""s.
The rear wheel position of MPV""s creates special demands on the tires because of the lighter and variable loading of the tire. The footprint of FIG. 2 depicts the same prior art tire of FIG. 1 when placed on the rear position at 50% load. At this position the centerline pressure of the footprint is highest and needs to resist abrasion. Larger tread elements are needed at the centerline to resist this higher abrasion tendency while still providing traction and hydroplaning resistance by providing open shoulders a design that is in direct conflict with the needs of the front position tire.
The requirements for an all wheel position tire require a balance of design characteristics to meet the performance needs of both front and rear tire applications. Inevitably,certain design tradeoffs or balances must be made in order to achieve performance levels for both positions. This results in a balanced design that cannot be fully optimized for either position. The Wrangler GS-A and Wrangler Aquatred represents the state of the art approach in balanced performance while still meeting the special needs of the front tire positions on MPV""s.
To push tire performance to the next level, new approaches to design and materials must be discovered. One attempt to push design towards the next level of tire performance for MPV""s light trucks and automobiles focuses on optimizing the functional design based on wheel position requirements. The front tire requirements have been addressed by the prior art but were not fully exploited because of all wheel position demands. The demands of the rear position have been studied and understood but until recently have not been fully optimized due to the requirements for all wheel position capability. This invention focuses on optimizing both front and rear positions by designing a combination of tires having a unique tire for each position front and rear. The front tires can be more fully optimized to resist fast outside shoulder wear while providing outstanding wet traction and handling. The rear position can be more fully optimized to resist the fast centerline wear associated with rear wheel drive and light loads while providing high levels of driving traction.
To optimize the front tire position, the invention employs multiple tread radii contouring the tread to achieve improved tread pressure distribution. Additionally, tread pattern mass is adjusted to enhance anti-hydroplaning performance while still providing resistance to outside shoulder wear. Since front tires encounter water on the road first, they must be more capable of anti-hydroplaning performance than the rear tires that run in their trough or wake.
To optimize the rear, tires position, the invention also employs multiple tread radii contouring the tread to achieve full contact of the tire footprint from shoulder to shoulder. This allows for more even tread across the tread with secondary benefits of tuition improvement though full tread pattern contact. This has been previously difficult to achieve because of the light loading of the rear position and the use of all wheel position design balances resulting in high centerline pressure and wear. Additionally, the tread mass distribution can now be optimized to resist fast centerline abrasion with tread design pattern details specifically suited for rear wheel drive traction.
Optimizing both front and rear designs based on functional requirements can result in tires that require no rotation and perform to higher levels of performance than tires that are designed for all wheel position use.
While it has been known in the art that specific tires for a particular wheel position can improve performance by the selection of certain distinct tread patterns that are wheel specific as is taught in German patent DE 3901624A1. These concepts have been limited to exotic racing type vehicles or high performance vehicles which may even employ different sized tires.
Similarly, another German Patent 1,480,962 teaches a combination of features wherein different front and rear tires are used.
The invention disclosed below employs not only a distinct tread mass distribution but also teaches a specific footprint shape factor at normal pressure for variable vehicle loading conditions which can be achieved by unique tread arc curvatures for the front position tires and the rear position tires.
A pneumatic radial tire combination for four-wheeled automobile or light truck vehicles has a pair of front steer position tires and a pair of rear position tires. Each front steer position tire or rear position tire has a footprint, each footprint has an axial width W, as measured at the lateral extremes of the footprint, a centerplane CP midway between the lateral extremes of the footprint.
The tire combination has a pair of front steer position tires and a pair of rear position tires. The front steer position tire has a footprint when the tire is normally inflated for normal load that has a footprint shape factor of greater than 1.00 at a 50% load, and about 1.00 at both 85% load and 100% load. The footprint shape factor is defined as the maximum circumferential extent of the tire""s footprint at the centerplane of the tire""s footprint width divided by the average of the circumferential extent of the tire""s footprint width as measured at 40% of the footprint from both sides of the central plane of the footprint.
The rear position tire footprint, when the tire is normally inflated for normal load, has a footprint shape factor of about 1.00 at 50% load, and 1.00 or less than 1.00 at 85% load and 100% load when measured similar to the method described for the front position tires.
The footprints of each tire is divided into a central region, and a first shoulder and a second shoulder region. The central region extends 20% of the footprint width W on either side of the centerplane CP. Each first and second shoulder region extends from a lateral edge of the footprint to the central portion. The tire combination has the footprint of the steer position tires having a tread contact area at normal inflation and load wherein the central region contact area is less than the contact area of a first or second shoulder area, while the footprint of the rear position tires have a contact area at normal inflation and load in the central region greater than the first or second shoulder portions.
Preferably, the central portion tread contact area of the front steer position tires is less than the first shoulder and the second shoulder contact areas respectively and the center portion of the tread contact area of the rear position tires is greater than the first and the second shoulder contact areas.
In a preferred embodiment, the central portion of the front steer position tires has a wide circumferential groove having a groove width of about 10% of the footprint width W at normal load and inflation while the rear position tire has two wide circumferential grooves, one wide groove being located between the contact area of the central portion and the contact area of each first and second shoulder area.
The inventive pneumatic radial tire combination for four-wheeled automobile or light truck vehicles has a pair of front steer position tires and a pair of rear position tires each tire having an axis of rotation and a casing. The casing has a carcass and a belt reinforcement radially outward of the carcass. A tread is radially outward of the belt structure of the casing. The tread has a pair of lateral edges, a tread arc extending between the lateral edges and a centerplane passing midway between the lateral edges and perpendicular to the axis of rotation.
The tire combination has a first tread arc for the front position tires and a second tread arc for the rear position tires, the tread arcs defined by a radially outer surface of the treads when the tires are normally inflated but unloaded.
The first tread arc has a curvature of maximum radius at the centerplane of the tire and a curvature of decreasing radius extending toward the lateral edges, at an intersection of the lateral edge and the tread arc a straight line drawn between the intersection of the tread arc curvature and the centerplane is inclined at an angle xcex8F of greater than 5xc2x0 relative to a tangent line L, L being tangent to the tread arc at the centerplane and parallel to the axis of rotation.
The second tread arc has a curvature of maximum radius at the centerplane of the tire and when measured similar to the tires for the front position has an angle xcex8R of less than 5xc2x0 relative to the tangent line L, L being tan gent to the second tread arc curvature at the centerplane and parallel to the axis of rotation.
In the preferred tire combination, the front tire position first tread arc curvature extending from the centerplane to a later;a edge has at least three radii of curvature R1, R2, R3 deceasing in size as the curvature extends from the centerplane to each lateral edge and the rear tire position second tread arc of curvature has at least two radii R1, R2 of curvature decreasing in size as the curvature extends from the centerplane to each lateral edge.
The front tire position radius R1 is the radius of curvature at the centerplane and R3 is the radius of curvature at the lateral edge. Most preferably R1 is greater than twice R3. R1 of the front tire position is preferably about, 600 mm and the tread radius R3 is greater than 200 mm.
The radii of curvature R1, R2, R3 of the front position tires have the following ratio of lateral width to arc width on at least one-half of the tread of less than 60% for R1, and greater than 20% for R2 and R3 respectively.
The rear tire position has the radius R1 preferably greater than 800 mm and the radius of curvature R2 of the rear position tire is less than 200 mm.
The radii of curvature R1 and R2 of the rear position tires have the following ratio of lateral width to arc width on at least one tread-half of greater than 50% for R1 and between 30% to 50% for R2.
xe2x80x9cAspect ratioxe2x80x9d of the tire means the ratio of its section height (SH) to its section width (SW) multiplied by 100% for expression as a percentage.
xe2x80x9cAsymmetric treadxe2x80x9d means a tread that has a tread pattern not symmetrical about the centerplane or equatorial plane EP of the tire.
xe2x80x9cCircumferentialxe2x80x9d means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction.
xe2x80x9cEquatorial plane (EP)xe2x80x9d means the plane passing midway between the width of the tread and perpendicular to the tire""s axis of rotation.
xe2x80x9cGroovexe2x80x9d means an elongated void area in a tread that may extend circumferentiary or laterally about the tread in a straight, curved, or zigzag manner. Circumferentially and laterally extending grooves sometimes have common portions. The xe2x80x9cgroove widthxe2x80x9d is equal to tread surface area occupied by a groove or groove portion, the width of which is in question, divided by the length of such groove or groove portion; thus, the groove width is its average width over its length. Grooves may be of varying depths in a tire. The depth of a groove may vary around the circumference of the tread, or the depth of one grove may be constant but vary from the depth of another groove in the tire. If such narrow or wide grooves are of substantially reduced depth as compared to wide circumferential grooves that they interconnect, they are regarded as forming xe2x80x9ctie barsxe2x80x9d tending to maintain a rib-like character in the tread region involved.
xe2x80x9cInboard sidexe2x80x9d means the side of the tire nearest the vehicle when the tire is mounted on a wheel and the wheel is mounted on the vehicle.
xe2x80x9cLateralxe2x80x9d means an axial direction.
xe2x80x9cNet contact areaxe2x80x9d means the total area of ground contacting elements between defined boundary edges divided by the gross area between the boundary edges as measured around the entire circumference of the tread.
xe2x80x9cNet-to-gross ratioxe2x80x9d means the total area of ground contacting tread elements between the lateral edges around the entire circumference of the tread divided by the gross area of the entire tread between the lateral edges.
xe2x80x9cNon directional treadxe2x80x9d means a tread that has no preferred direction of forward travel and is not required to be positioned on a vehicle in a specific wheel position or positions to ensure that the tread pattern is aligned with the preferred direction of travel. Conversely, a directional tread pattern has a preferred direction of travel rewiring specific wheel positioning. xe2x80x9cAxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d means lines or directions that are parallel to the axis of rotation of the tire.
xe2x80x9cOutboard sidexe2x80x9d means the side of the tire farthest away from the vehicle when the tire is mounted on a wheel and the wheel is mounted on the vehicle.
xe2x80x9cRadialxe2x80x9d and xe2x80x9cradiallyxe2x80x9d means directions radially toward or away from the axis of rotation of the tire.
xe2x80x9cRibxe2x80x9d means a circumferentially extending strip of rubber on the tread which is defined by at least one circumferential groove and either a second such groove or a lateral edge, the strip being laterally undivided by full-depth grooves.
xe2x80x9cSipexe2x80x9d means small slots molded into the tread elements of the tire that subdivide the tread surface and improve traction.
xe2x80x9cTread elementxe2x80x9d or xe2x80x9ctraction elementxe2x80x9d means a rib or a block element.